In Britain, protesters rolled the statue of Edward Colston into a deep river. It is unlikely to return to its plinth in Bristol.

Colston was an official in the Royal African Company which traded in human beings - slaves. Bristol has a sizable black population, many of whom are descended from slaves.

In the Deep South of the United States, statues of Confederate leaders are being removed, either by the authorities or by activists.

Confederates fought to the death for slavery, and the descendants of slaves don't see why the oppressors of their ancestors should be honoured.

The movement is gathering momentum in Australia, with a petition to remove the statue of Australia's first prime minister from its site in Port Macquarie.

The organisers of the petition argue that Sir Edmund Barton's government drafted the racist Immigration Restriction Act, which led to the White Australia Policy.

On top of that, the statue is on an Aboriginal burial ground.

There is a line of candidates.

Near the top of the list would be King Leopold of Belgium, whose rule of the Belgian Congo in the 19^th century involved unspeakable brutality, including the mutilation and execution of workers who failed to meet their quota for collecting rubber.

There is an utterly chilling historical photograph titled "Father stares at the hand and foot of his five-year-old, severed as a punishment for failing to make the daily rubber quota, Belgian Congo, 1904".

The word "assassin" has been daubed on the ruler's statue in Brussels.

In Britain, activists at Oxford University want the statue of Cecil Rhodes removed.

Rhodes was responsible for the colonisation of much of southern Africa but used some of his fortune to fund the educational scholarships to Oxford which bear his name, and from which many Australians have benefited.

And in Australia, the statue of James Cook was daubed with the words "No pride in genocide".

On the one hand, nobody argues that statues of Hitler should have been allowed to stand after the war.

Streets in East Berlin named after Stalin were renamed when the tyrant was finally denounced by the new (still tyrannous) leaders of the Soviet Union.

The toppling of the statue of the butcher Sadaam Hussein was celebrated.

But what about other despots, or simply leaders with blemished pasts?

After all, many leaders are neither monsters nor saints - Churchill was without doubt a racist but also played a leading role in the defeat of murderous racism.

After the recent defilement of his statue in London, Treasurer Josh Frydenberg asked: "If Churchill was a racist, what would you call the guy he stopped?"

Defenders of statues argue that the past was the past. There were different rules about what was right and wrong then.

Prime Minister Scott Morrison said this week: "I've always said we've got to be honest about our history. We've got to acknowledge the positive and the negative.

"But, you know, I think we've also got to respect our history as well. And this is not a licence for people to just go nuts on this stuff."

On the Cook statue in Hyde Park in Sydney, he pointed out that the explorer was actually an enlightened figure for his time (though not for our time). He wasn't a slaver.

"Australia when it was founded as a settlement, as NSW, was on the basis that there'd be no slavery. And while slave ships continued to travel around the world, when Australia was established yes, sure, it was a pretty brutal settlement.

"My forefathers and foremothers were on the First and Second Fleets. It was a pretty brutal place, but there was no slavery in Australia."

An ANU historian, Professor Bruce Scates, cites the Explorers' Monument in Fremantle which commemorates three white explorers killed in 1864. Plaques claimed that the men were "killed in their sleep" by "treacherous natives".

If there were a candidate for removal, you might think this would be it.

But after a lot of debate involving Aboriginal people, the original was kept - but with information about the provocation that led to the deaths.

Statues can be adapted to our times. Professor Scates wondered, for example, if the graffiti on the Cook statue shouldn't have been left there.

There is no blanket rule for all statues, though unmitigated evil should not be celebrated. Monsters get no statues.

But for others, a set of criteria should apply. The British philosopher and journalist David Edmonds says: "One may be whether the views or actions of the figure in question were typical for their time. If so, that could make them less blameworthy.

"Another is the extent of their misdeeds and how that is evaluated against their achievements. Churchill held opinions that would disbar him from political office today - despicable, yes, but surely massively outweighed by the scale of his accomplishments."

And finally, there is the question of how offensive a statue is to people today. "How does looking at the statue make passersby feel? This, in turn, will be connected to whether the history still resonates - an ancient statue of some medieval warlord, however bloody and brutal his conquests, probably won't bother anybody."

Probably a lot of discussion - but that can only work if there is goodwill. Professor Scates is sure that the descendants of the original occupiers of land on which a statue stands should be involved: "I look at it as an opportunity for dialogue and education."